Optical modulation schemes such as DPSK (i.e., differential binary phase shift keying) modulation, DQPSK (i.e., differential quadrature phase shift keying) modulation, and the like have been attracting attention as a technology that is capable of achieving a high-bit-rate optical transmission in the range of 40 G/s per wavelength or more. A receiver used in phase modulation schemes such as DPSK and DQPSK employs a delay interferometer for detecting phase information.
A delay interferometer uses two optical beams to interfere with each other, and a phase difference between these two beams may be stabilized with precision in the range of 1/100 of the wavelength. Typically, temperature-based control is utilized to adjust a phase difference. For example, a phase compensator that is made of a material having a thermo-optical effect such as refractive-index-temperature dependency is known to be used in a delayed interferometer.
However, phase adjustment based on temperature control may have a slow response speed because heat is applied to a material having a thermo-optical effect for expansion.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2007-306371